1. Technical Field of the Invention
This invention relates generally to wireless communication systems and more particularly to antennas used in such systems.
2. Description of Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks to radio frequency identification (RFID) systems. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, radio frequency (RF) wireless communication systems may operate in accordance with one or more standards including, but not limited to, RFID, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), WCDMA, local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), LTE, WiMAX, and/or variations thereof. As another example, infrared (IR) communication systems may operate in accordance with one or more standards including, but not limited to, IrDA (Infrared Data Association).
Depending on the type of RF wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, RFID reader, RFID tag, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the Internet, and/or via some other wide area network.
For each RF wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the receiver is coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives inbound RF signals via the antenna and amplifies then. The one or more intermediate frequency stages mix the amplified RF signals with one or more local oscillations to convert the amplified RF signal into baseband signals or intermediate frequency (IF) signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out of band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with the particular wireless communication standard.
As is also known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with a particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
Since the wireless part of a wireless communication begins and ends with the antenna, a properly designed antenna structure is an important component of wireless communication devices. As is known, the antenna structure is designed to have a desired impedance (e.g., 50 Ohms) at an operating frequency, a desired bandwidth centered at the desired operating frequency, and a desired length (e.g., ¼ wavelength of the operating frequency for a monopole antenna). As is further known, the antenna structure may include a single monopole or dipole antenna, a diversity antenna structure, the same polarization, different polarization, and/or any number of other electro-magnetic properties.
One popular antenna structure for RF transceivers is a three-dimensional in-air helix antenna, which resembles an expanded spring. The in-air helix antenna provides a magnetic omni-directional monopole antenna. Other types of three-dimensional antennas include aperture antennas of a rectangular shape, horn shaped, etc.; three-dimensional dipole antennas having a conical shape, a cylinder shape, an elliptical shape, etc.; and reflector antennas having a plane reflector, a corner reflector, or a parabolic reflector. An issue with such three-dimensional antennas is that they cannot be implemented in the substantially two-dimensional space of a substrate such as an integrated circuit (IC) and/or on the printed circuit board (PCB) supporting the IC.
Two-dimensional antennas are known to include a meandering pattern or a micro strip configuration. For efficient antenna operation, the length of an antenna should be ¼ wavelength for a monopole antenna and ½ wavelength for a dipole antenna, where the wavelength (λ)=c/f, where c is the speed of light and f is frequency. For example, a ¼ wavelength antenna at 900 MHz has a total length of approximately 8.3 centimeters (i.e., 0.25*(3×108 m/s)/(900×106 c/s)=0.25*33 cm, where m/s is meters per second and c/s is cycles per second). As another example, a ¼ wavelength antenna at 2400 MHz has a total length of approximately 3.1 cm (i.e., 0.25*(3×108 m/s)/(2.4×109 c/s)=0.25*12.5 cm).
Regardless of whether a two-dimensional antenna is implemented on an IC and/or a PCB, the amount of area that it consumes is an issue. For example, a dipole antenna that uses Hilbert shapes operating in the 5.5 GHz frequency band requires each antenna element to be ¼ wavelength, which is 13.6 mm [“Compact 2D Hilbert Microstrip Resonators,” MICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Vol. 48, No. 2, February 2006]. Each antenna element consumes approximately 3.633 mm2 (e.g., ½*(1.875 mm×3.875 mm)), which has a length-to-area ratio of 3.74:1 (e.g., 13.6:3.633). While this provides a relatively compact two-dimensional antenna, further reductions in consumed area are needed with little or no degradation in performance.